The present invention relates to molecular beam epitaxy (MBE) systems.
In molecular beam epitaxy, thin film deposition is achieved by directing molecular beams onto a substrate in an ultra high vacuum. Preferably the beams are not ionized, but are neutral molecular or atomic species, whence the name of the process. The substrate is heated to a temperature where kT is large enough to permit a deposited atom to move laterally for an average distance of at least several angstroms, to permit the deposited atoms to find their energetically preferred sites. Thus, MBE permits growth of thin films with extremely high crystalline quality. The MBE technique is generally well known, and has been widely discussed. See, for example, the following review articles, which are hereby incorporated by reference:
A. Y. Cho and J. R. Arthur, in Progress in Solid State Chemistry, edited by J. McCaldin and G. Somorjai (Pergamon, New York, 1975), Vol. 10, p. 157; PA1 L. L. Chang, in Handbook on Semiconductors, edited by S. P. Keller (North-Holland, Amsterdam, 1980), Vol. 3 Chapter 9; PA1 C. E. C. Wood, in Physics of Thin Films, edited by C. Haff and M. Frankcombe (Academic, New York, 1980), Vol. 11, p.35; PA1 C. T. Foxon and B. A. Joyce, in Current Topics in Materials Science, edited by E. Kaldis (North-Holland, Amsterdam, 1981), Vol. 7, Chapter 1. PA1 A molecular beam epitaxy system comprising: PA1 A vacuum growth chamber comprising a substrate support and a plurality of effusion sources; PA1 means for exhausting said growth chamber to ultrahigh vacuum; PA1 a second chamber operatively connected to said growth chamber, said second chamber comprising a vacuum chamber connected to a second means for exhausting said second chamber to ultrahigh vacuum; PA1 means for transferring wafers between said growth chamber and said second chamber; and PA1 a source outgassing mount mounted on said analysis chamber, said source outgassing mount comprising a vacuum flange adapted to receive one of said sources for outgassing.
Molecular beam epitaxy is very attractive as a product technology for many applications, due to its unique capabilities. MBE easily produces hetero-epitaxial structures, wherein an epitaxial layer of one material is epitaxially deposited onto an underlying layer of a different material. The abrupt epitaxial transitions which can thus be achieved can be rapidly alternated to achieve superlattice structures, wherein, as the different epitaxial layers become very thin, some anomalous and highly desirable properties appear. Such structures are very difficult to make in any other way. MBE can also be used to make strained superlattices, wherein materials which are not lattice-matched in isolation are nevertheless grown in a perfect epitaxial structure. That is, materials which have the same crystal structure, but which would not have the same lattice spacing normally, can not be grown epitaxially by conventional methods. For example, the lattice constant of InAs.sub.0.4 Sb.sub.0.6 is 0.4% less than that of InAs.sub.0.27 Sb.sub.0.73. Thus, if one attempts to grow an epitaxial layer of InAs.sub.0.4 Sb.sub.0.6 on a InAs.sub.0.27 Sb0.73 substrate by conventional methods such as chemical vapor deposition, the two lattices would not be matched. That is, it is desirable to have the interface between the two materials preserve the crystalline structure of the materials, so that the first lattice is a smooth continuation of the second lattice, except that more arsenic atoms and fewer antimony atoms are now found on the Group V sites. This can not be achieved by conventional methods, but is readily achieved in superlattice structures by MBE. MBE also promises other unique capabilities, such as epitaxial deposition of insulators over semiconductors, metals over insulators, etc.
However, attractive as these capabilities are for semiconductor device fabrication, MBE systms at present are primarily a laboratory tool rather than a production tool, simply because the throughput of MBE systems is slow. In part, the slow throughput of MBE systems is unavoidable, since it is difficult to achieve good quality deposited material if the deposition rates used are greater than several microns per hour. However, in large part this problem of slow throughput has been due to the difficulties of wafer handling.
Thus, it is an object of the present invention to provide an MBE system having reduced time requirements for wafer handling.
A problem in conventional MBE systems is outgassing of the sources. As shown in FIG. 3, a conventional MBE evaporation source is a small crucible (in which the source material will be placed), mounted on a vacuum flange together with a resistive heater, a heat shield, and a thermocouple. This structure may contain volatile contaminants, which are likely to escape when the source is heated to the temperatures used for evaporation of the source material.
Therefore, for best quality MBE growth, it has been found desirable to outgas the source, before the source material is actually placed in the crucible, at a temperature of about 1400 C. or higher for at least several dozen hours. After the source material is placed in the crucible, a second bakeout step, at about 50.degree. over the source evaporation temperature, is performed for a shorter period of about 1 hour. A short exposure to air subsequent to these outgassing steps is not harmful, since these steps are not directed merely at adsorbed water and other low-temperature contaminants, but are directed at removing the high-temperature contaminants which may initially be present in the crucible and in the material of the source structure.
However, while this source outgassing provides better quality grown material, it is obviously quite time consuming. In particular, since the outgassing must be performed under high vacuum conditions, it could be performed with a source in place in the growth chamber of the MBE system, but this would obviously tie up the growth chamber of the MBE system for extended periods and therefore further degrade the already low throughput of the MBE system.
It would be possible to provide a separate high vacuum system for outgassing the sources, but this would obviously be expensive, not only in capital cost but also in technician time, due to the system bakeout and other routine maintenance steps which are periodically necessary for any operating ultra high vacuum system.
Thus, it is an object of the present invention to provide a molecular beam epitaxy system which includes means for outgassing sources without degrading throughput of the system.
It is a further object of the present invention to provide a MBE system which incorporates means for outgassing molecular beam sources, without degrading throughput of the system and without requiring any additional vacuum system.
The present invention provides this objective by providing a molecular beam epitaxy system which includes, as is conventional, more than one separate ultra high vacuum chamber. That is, a growth chamber is separated from a sample analysis chamber by a vacuum valve, through which wafers can be passed and which can be closed to isolate the growth chamber from the analysis chamber. In the present invention, the analysis chamber includes a source outgassing fixture, into which one source can be temporarily attached, so that source outgassing can be performed in the secondary chamber, after a wafer has been loaded into the growth chamber, while growth is proceeding in the growth chamber. Thus, no additional ultra high vacuum facility is required, but source outgassing can be performed with no degradation of throughput.
According to the present invention there is provided: